<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>五角星</title>
    <style>
        .star {
            position: relative;
            margin: 200px auto;
            width: 0;
            height: 0;
            /* border-style  表示边框 */
            border-style: solid; 
            border-color: transparent transparent red transparent;
            /* border-width 属性可以设置盒子模型的边框宽度。 */
            border-width: 70px 100px;
            transform: rotate(35deg);
        }
        .star::before {
            position: absolute;
            content: '';
            width: 0;
            height: 0;
            top: -128px;
            left: -95px;
            border-style: solid;
            border-color: transparent transparent red transparent;
            border-width: 80px 30px;
            transform: rotate(-35deg);
        }
        .star::after {
            position: absolute;
            content: '';
            width: 0;
            height: 0;
            top: -45px;
            left: -140px;
            border-style: solid;
            border-color: transparent transparent red transparent;
            border-width: 70px 100px;
            transform: rotate(-70deg);
        }
    </style>
</head>
<body>
    <div class="star"></div>
</body>
</html>